Hydrogen bonded supramolecular architectures of organic salts based on 5,7-dimethyl-1,8-naphthyridine-2-amine and acidic compounds

Article abstract of DOI:10.1016/j.molstruc.2010.04.006

Studies concentrating on hydrogen bonding between the base of 5,7-dimethyl-1,8-naphthyridine-2-amine and acidic compounds have led to an increased understanding of the role 5,7-dimethyl-1,8-naphthyridine-2-amine has in binding with acidic compounds. Here

Full text of DOI:10.1016/j.molstruc.2010.04.006

Journal of Molecular Structure 975 (2010) 128–136
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Hydrogen bonded supramolecular architectures of organic salts based on
5,7-dimethyl-1,8-naphthyridine-2-amine and acidic compounds
b
a
c
d
a
a
Shouwen Jin a, , Wenbiao Zhang , Li Liu , Hongfang Gao , Daqi Wang , Rongpo Chen , Xiaolei Xu
*
a Faculty of Science ZheJiang Forestry University, Lin’An 311300, PR China
b School of Engineering ZheJiang Forestry University, Lin0An 311300, PR China
c Middle School of JinShan, Shanyin Road 1268 XiaoShan, Hangzhou ZheJiang 311203, PR China
d Department of Chemistry, Liaocheng University, Liaocheng 252059, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Studies concentrating on hydrogen bonding between the base of 5,7-dimethyl-1,8-naphthyridine-2-
amine and acidic compounds have led to an increased understanding of the role 5,7-dimethyl-1,8-naph-
thyridine-2-amine has in binding with acidic compounds. Here anhydrous and hydrated multicomponent
crystals of 5,7-dimethyl-1,8-naphthyridine-2-amine have been prepared with oxalic acid, 2,4,6-trinitro-
phenol, terephthalic acid, and phthalic acid. The four crystalline forms reported are organic salts of which
the crystal structures have all been determined by X-ray diffraction. All products were formed in solution
and obtained by the slow evaporation technique. The role of weak and strong hydrogen bonding in the
crystal packing is ascertained.
Received 2 November 2009
Received in revised form 8 April 2010
Accepted 8 April 2010
Available online 13 April 2010
Keywords:
Synthesis
Structure characterization
Hydrogen bonding
Naphthyridine derivatives
Acidic compounds
Crown Copyright Ó 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction
is intermolecular hydrogen bonding. As the 1,8-naphthyridine core
(N^C^N) has similar dimensions to those of carboxylate (O^C^O),
Nowadays, hydrogen bonding has been widely developed in the
area of crystal engineering, supramolecular chemistry, material
science, and biological recognition [1–4]. The application of inter-
molecular hydrogen bonds is a well known and efficient tool to
regulate the molecular arrangement in a crystal structure [4].
Through hydrogen bonds we can form co-crystals and organic
salts. In pharmaceuticals, salt formation is often used in order to
modify the properties of the compounds [5]. Salt formation can
be used to increase or decrease solubility, to improve stability
and to reduce hygroscopicity of a drug product. There are many
interesting hydrogen bonded topological structures from infinite
1D chain to 3D supramolecular framework [6,7]. The carboxylic
acid contains the important hydrogen bonding functional group
for crystal engineering [8]. Carboxylic acids aggregate in the solid
state as dimer, catemer, and bridged motifs [9]. It is interesting
to exploit the robust and directional recognition of carboxylic acids
with N-heterocyclic moieties [10].
the 5,7-dimethyl-1,8-naphthyridine-2-amine may act as a poten-
tially tridentate ligand. The binary organic salts of the carboxylic
acids and 5,7-dimethyl-1,8-naphthyridine-2-amine may display
the different hydrogen-bonding patterns from the three different
N atoms. As an extension of our study of weak interactions (hydro-
gen bonding,
p–p interaction, and halogen bonding) concerning
1,8-naphthyridine derivatives [13], herein we report the prepara-
tion and structures of four organic salts assembled from 5,7-di-
methyl-1,8-naphthyridine-2-amine (L) and the corresponding
acidic compounds (Scheme 1), respectively. The four organic salts
are 5,7-dimethyl-1,8-naphthyridine-2-amine: (oxalic acid)0.5 (1)
[HLꢀ(ox2ꢁ
0.5ꢀH2O, ox = oxalate], 5,7-dimethyl-1,8-naphthyridine-
)
2-amine: picric acid (2) [HLꢀ(pic), pic = picrate], (5,7-dimethyl-
1,8-naphthyridine-2-amine)2: terephthalic acid (3) [HL+ꢀLꢀ(tp2ꢁ
0.5ꢀ
)
(H2tp)0.5, tp = terephthalate], and (5,7-dimethyl-1,8-naphthyri-
dine-2-amine)2: phthalic acid (4) [HL+ꢀLꢀ(Hopꢁ), Hopꢁ = hydrogen
phthalate] (Scheme 2).
Recently 1,8-naphthyridine derivatives have been reported to
form supramolecular compounds under the multiple hydrogen
bonding action [11]. The derivatives of 1,8-naphthyridine have also
been widely utilized as molecular recognition receptors for urea,
carboxylic acids and guanine [12], in which the major driving force
2. Experimental section
2.1. Materials and physical measurements
The chemicals and solvents used in this work are of analytical
grade and available commercially and were used without further
purification. 5,7-Dimethyl-1,8-naphthyridine-2-amine was pre-
* Corresponding author. Tel./fax: +86 571 6374 0010.
E-mail address: Jinsw@zjfc.edu.cn (S. Jin).
0022-2860/$ - see front matter Crown Copyright Ó 2010 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2010.04.006

S. Jin et al. / Journal of Molecular Structure 975 (2010) 128–136
129
1602m, 1580s(
m
as(COO)), 1540s, 1500m, 1487m, 1448m, 1400m,
(CAO)), 1240m, 1200m, 1160m,
1380s( s(COO)), 1295m, 1288s(
m
m
1080m, 1000m, 930m, 850m, 740s, 700m, 646s, 610m, 540m,
480m, 440m.
2.2.2. b. 5,7-Dimethyl-1,8-naphthyridine-2-amine: (2,4,6-
trinitrophenol) (2)
To a methanol solution (5 ml) of 5,7-dimethyl-1,8-naphthyri-
dine-2-amine (34.8 mg, 0.2 mmol) was added 2,4,6-trinitrophenol
(46 mg, 0.2 mmol) in 10 ml methanol. The solution was filtered
immediately. Pale yellow block crystals were isolated after
several minutes from the solution at room temperature in air. The
crystals were collected and dried in air to give the title compound
[HLꢀ(pic)] (2) Yield: 72 mg, 89.92%. m. p. 202–203 °C. Anal. calcd
for C16H14N7O6: C, 47.96; H, 3.50; N, 24.48. Found: C, 47.90; H,
Scheme 1. Hydrogen bond synthons discussed in this paper.
3.44; N, 24.46. Infrared spectrum (KBr disc, cmꢁ1): 3440s(
3240s( s(NH)), 3190s, 3100s, 2940m, 2880m, 2848m, 2820m,
2370m, 2330m, 1740m, 1665s, 1614s, 1600s, 1560s, 1520s(mas
(NO2)), 1480m, 1440m, 1360s, 1320s( s(NO2)), 1260s, 1240m,
mas(NH)),
pared by the method described in the literature [14]. The FT-IR
spectra were recorded from KBr pellets in range 4000–400 cmꢁ1
on a Mattson Alpha-Centauri spectrometer. Microanalytical (C, H,
N) data were obtained with a Perkin-Elmer Model 2400II elemental
analyzer. Melting points of new compounds were recorded on an
XT-4 thermal apparatus without correction.
m
m
1210m, 1160m, 1080m, 1020m, 980m, 920m, 880m, 840m, 820m,
790m, 720m, 680m, 660m, 620m, 540m, 460m.
2.2. Preparation of the compounds 1–4
2.2.3. c. (5,7-Dimethyl-1,8-naphthyridine-2-amine)2: (terephthalic
acid) (3)
2.2.1. a. 5,7-Dimethyl-1,8-naphthyridine-2-amine: (oxalic acid)0.5
H2O (1)
:
5,7-Dimethyl-1,8-naphthyridine-2-amine (34.8 mg, 0.2 mmol)
was dissolved in 5 ml of ethanol. To this solution was added tere-
phthalic acid (34 mg, 0.2 mmol) in 3 ml DMSO. Colorless prisms
were afforded after 1 week of slow evaporation of the solvent.
The crystals were collected and dried in air to give the title com-
To an ethanol solution (8 ml) of 5,7-dimethyl-1,8-naphthyri-
dine-2-amine (34.8 mg, 0.2 mmol) was added oxalic acid dihydrate
(25.2 mg, 0.2 mmol). The solution was stirred for a few minutes,
then the solution was filtered. The solution was left standing at
room temperature for several days, colorless crystals were isolated
after slow evaporation of the ethanol solution in air. The crystals
were collected and dried in air to give the title compound
pound [HL+ꢀLꢀ(tp2ꢁ
0.5ꢀ(H2tp)0.5] (3) yield 70 mg, 68.28%. m. p.
)
196–198 °C. Elemental analysis performed on crystals exposed to
the atmosphere: calc. for C28H28N6O4: C, 65.55; H, 5.46; N, 16.39.
Found: C, 65.54; H, 5.41; N, 16.32. Infrared spectrum (KBr disc,
[HLꢀ(ox2ꢁ
)
0.5ꢀH2O] (1) Yield: 39 mg, 82.54%. m. p. 189–191 °C. Anal.
cmꢁ1): 3460m (broad,
m
(OH)), 3340s(
3064m, 2950m, 2880m, 2810m, 2720m, 2560w, 2380m, 1940w,
1860w, 1820w, 1700w, 1660s, 1645s( (C@O)), 1620s, 1580s(mas
(COO)), 1560m, 1530s, 1460m, 1420m, 1380s( s(COO)), 1300m,
1284m( (CAO)), 1240m, 1200m, 1160m, 1080m, 1020m, 960m,
mas(NH)), 3160s(ms(NH)),
calcd for C11H14N3O3: C, 55.87; H, 5.93; N, 17.78. Found: C, 55.82; H,
5.91; N, 17.73. Infrared spectrum (KBr disc, cmꢁ1): 3360s(
broad), 3306s( as(NH)), 3140s( s(NH)), 3070s, 2980s, 2920s,
2380m, 2320m, 2080w, 1760w, 1670s, 1640s( (C@O)), 1620s,
m
(H2O),
m
m
m
m
m
m
Scheme 2. The four organic salts described in this paper, 1–4.

130
S. Jin et al. / Journal of Molecular Structure 975 (2010) 128–136
920m, 890m, 820m, 800m, 780m, 760m, 740m, 660m, 620m,
560m, 520m, 460m, 440m, 420m.
3. The IR bands which indicate chemical functionalities in the
resulting compounds are given in Table 4.
3. Results and discussion
2.2.4. d. (5,7-Dimethyl-1,8-naphthyridine-2-amine)2: (phthalic acid)
(4)
3.1. Preparation and general characterization
5,7-Dimethyl-1,8-naphthyridine-2-amine (34.8 mg, 0.2 mmol)
was dissolved in 5 ml of ethanol. To this solution was added phtha-
lic acid (34 mg, 0.2 mmol) in 5 ml ethanol. Colorless prisms were
afforded after several weeks of slow evaporation of the solvent.
The crystals were dried in air to give the title compound
[HL+ꢀLꢀ(Hopꢁ)] (4), yield 72 mg, 70.24%. m. p. 180–182 °C.
Elemental analysis performed on crystals exposed to the
atmosphere: calc. for C28H28N6O4: C, 65.55; H, 5.46; N, 16.39.
Found: C, 65.52; H, 5.44; N, 16.37. Infrared spectrum (KBr disc,
5,7-Dimethyl-1,8-naphthyridine-2-amine has good solubility in
common organic solvents, such as CH3OH, C2H5OH, CH2Cl2, CHCl3,
and CH3CN. The crystals were grown by slow evaporation of the
corresponding polar solution at room temperature.
The preparation of compounds 1–4 were carried out with 5,7-
dimethyl-1,8-naphthyridine-2-amine and the corresponding acidic
component in 1: 1 ratio. The four compounds are not hygroscopic,
and they all crystallized with no solvent molecules accompanied
except compound 1. The molecular structures and their atom
labelling schemes for the four structures are illustrated in Figs. 1,
3, 5 and 7, respectively.
In the preparation of the organic salts 1, 2, 3, and 4, the acidic
compounds were mixed directly with the base in the correspond-
ing solution, which was allowed to evaporate at ambient condi-
tions to give the final crystalline products. The elemental analysis
data for the four compounds are in good agreement with their
compositions. The infrared spectra of the four compounds are con-
sistent with their chemical formulas determined by elemental
analysis and further confirmed by X-ray diffraction analysis. H
atoms connected to O or N atoms were well found from the differ-
ence electron density map, which also indirectly confirms the pro-
ton transfer.
In 1 and 2, the protons of each acidic molecule have transferred
to the 5,7-dimethyl-1,8-naphthyridine-2-amine molecules. In 3,
both protons of one carboxylic acid have transferred to the base,
while another carboxylic acid remains protonized. In 4, only one
proton of the phthalic acid has transferred to the 5,7-dimethyl-
1,8-naphthyridine-2-amine molecule, thus 3 and 4 form 2:1 salts.
But the acid presents a different valence number in these two sit-
uations, ꢁ2 for compound 3, and ꢁ1 for compound 4.
cmꢁ1): 3560s(
m(OH)), 3440s (multiple, mas(NH)), 3320s(ms(NH)),
3140m, 3060m, 2990m, 2920m, 2812m, 2720m, 2540w, 2360m,
2190m, 1960w, 1840w, 1800w, 1775w, 1730m, 1670s,
1648s(
m
(C@O)), 1628s, 1580s(
m
as(COO)), 1500s, 1460s, 1426s,
(CAO)), 1240m, 1196m,
1380s(
m
s(COO)), 1350s, 1300m, 1280s(
m
1130m, 1100m, 1060m, 1020m, 930m, 888m, 800m, 780m, 740m,
680m, 660m, 620m, 580m, 540m, 510m, 470m, 450m.
2.3. X-ray crystallography and data collection
Suitable crystals were mounted on a glass fiber on a Bruker
SMART 1000 CCD diffractometer operating at 50 kv and 40 mA
using Mo Ka radiation (0.71073 Å). Data collection and reduction
were performed using the SMART and SAINT software [15]. The
structures were solved by direct methods, and the non-hydrogen
atoms were subjected to anisotropic refinement by full-matrix
least squares on F2 using SHELXTL package [16].
Hydrogen atom positions for all of the structures were located
in a difference map and refined independently. Further details of
the structural analysis are summarized in Table 1. Selected bond
lengths and angles for the salts 1, 2, 3, and 4 are listed in Table
2, the relevant hydrogen bond parameters are provided in Table
Table 1
Summary of X-ray crystallographic data for complexes 1, 2, 3, and 4.
1
2
3
4
Formula
Fw
C
11H14N3O3
C16H14N7O6
400.34
C28H28N6O4
512.56
C28H28N6O4
512.56
236.25
T, K
298(2)
298(2)
298(2)
298(2)
Wavelength, Å
Crystal system
Space group
a, Å
b, Å
c, Å
0.71073
Monoclinic
P2(1)/n
7.3486(14)
10.4028(18)
14.739(2)
90
89.8960(10)
90
1126.7(3)
4
1.393
0.104
500
0.42 ꢂ 0.40 ꢂ 0.39
2.40–25.02
ꢁ8 6 h 6 8
ꢁ12 6 k 6 11
ꢁ17 6 l 6 9
5220
0.71073
Triclinic
P-1
7.9058(10)
8.0659(11)
14.0204(17)
83.3330(10)
84.249(2)
83.9670(10)
879.6(2)
0.71073
Triclinic
P-1
0.71073
Triclinic
P-1
7.7530(10)
12.3651(13)
13.5561(15)
100.946(2)
100.291(2)
92.5200(10)
1251.3(3)
2
1.360
0.094
540
0.41 ꢂ 0.33 ꢂ 0.26
1.56–25.02
ꢁ7 6 h 6 9
ꢁ12 6 k 6 14
ꢁ16 6 l 6 16
6510
7.8990(10)
11.4371(14)
14.5669(16)
97.7550(10)
100.522(2)
103.618(2)
1235.5(3)
2
1.378
0.095
540
0.41 ꢂ 0.18 ꢂ 0.16
1.86–25.02
ꢁ9 6 h 6 9
ꢁ13 6 k 6 13
ꢁ15 6 l 6 17
6535
a
, deg.
b, deg.
c
, deg.
V, Å3
Z
2
Dcalcd, Mg/m3
Absorption coefficient, mmꢁ1
F(0 0 0)
1.512
0.119
414
0.46 ꢂ 0.43 ꢂ 0.30
1.47–25.02
ꢁ9 6 h 6 9
ꢁ9 6 k 6 7
ꢁ15 6 l 6 16
4598
Crystal size, mm3
h Range, deg
Limiting indices
Reflections collected
Reflections independent (Rint
)
1976 (0.0395)
1.018
3061 (0.0197)
1.008
4335 (0.0255)
1.010
4309 (0.0276)
0.864
Goodness-of-fit on F2
R indices [I > 2
r
I]
0.0507, 0.1168
0.0917, 0.1440
0.203, ꢁ0.260
0.0464, 0.1143
0.0882, 0.1457
0.164, ꢁ0.187
0.0519, 0.1167
0.1135, 0.1571
0.198, ꢁ0.170
0.0554, 0.1243
0.1287, 0.1499
0.203, ꢁ0.248
R indices (all data)
Largest diff. peak and hole, e.Åꢁ3

S. Jin et al. / Journal of Molecular Structure 975 (2010) 128–136
131
Table 2
Table 3
Selected bond lengths [Å] and angles [°] for compounds 1, 2, 3, and 4.
Hydrogen bond distances and angles in studied structures of 1, 2, 3, and 4.
1
DAHꢀ ꢀ ꢀA
d(DAH)
[Å]
d(Hꢀ ꢀ ꢀA)
d(Dꢀ ꢀ ꢀA)
<(DHA)
[°]
N(1)AC(2)
N(2)AC(6)
N(3)AC(2)
O(2)AC(1)
C(6)AN(2)AC(7)
N(3)AC(2)AN(1)
1.340(3)
1.336(3)
1.312(3)
1.232(3)
116.5(2)
119.9(2)
N(1)AC(6)
N(2)AC(7)
O(1)AC(1)
C(2)AN(1)AC(6)
O(2)AC(1)AO(1)
N(2)AC(6)AN(1)
1.378(3)
1.345(3)
1.251(3)
123.9(2)
125.7(3)
115.8(2)
[Å]
[Å]
1
O(3)AH(3F)ꢀ ꢀ ꢀN(2)
0.85
2.16
2.52
1.88
1.94
2.65
1.99
1.81
2.972(3)
3.117(3)
2.690(3)
2.738(3)
3.285(3)
2.840(3)
2.656(3)
159.3
127.9
158.4
154.5
132.1
170.7
167.6
O(3)AH(3E)ꢀ ꢀ ꢀO(2)#1 0.85
O(3)AH(3E)ꢀ ꢀ ꢀO(1)
0.85
N(3)AH(3B)ꢀ ꢀ ꢀO(3)#2 0.86
2
N(3)AH(3A)ꢀ ꢀ ꢀO(1)
N(3)AH(3A)ꢀ ꢀ ꢀO(2)
N(1)AH(1)ꢀ ꢀ ꢀO(1)
0.86
0.86
0.86
N(1)AC(1)
N(2)AC(6)
N(3)AC(1)
N(4)AO(2)
1.335(3)
1.340(3)
1.332(3)
1.212(3)
1.220(3)
1.442(3)
1.215(3)
1.256(3)
123.7(2)
117.6(3)
122.3(2)
119.5(3)
119.5(3)
117.7(3)
N(1)AC(5)
N(2)AC(5)
N(4)AO(3)
N(4)AC(12)
N(5)AO(5)
N(6)AO(7)
N(6)AC(16)
C(1)AN(1)AC(5)
O(3)AN(4)AO(2)
O(2)AN(4)AC(12)
O(4)AN(5)AC(14)
O(7)AN(6)AO(6)
O(6)AN(6)AC(16)
N(1)AC(5)AN(2)
1.341(3)
1.354(3)
1.205(3)
1.463(3)
1.221(3)
1.201(3)
1.450(3)
116.8(2)
123.7(3)
118.7(3)
118.3(3)
122.4(3)
118.0(3)
116.5(2)
2
N(3)AH(3B)ꢀ ꢀ ꢀO(1)#1 0.86
2.29
2.23
2.00
2.887(3)
3.079(4)
2.801(3)
126.5
171.8
154.4
N(5)AO(4)
N(5)AC(14)
N(6)AO(6)
O(1)AC(11)
C(6)AN(2)AC(5)
O(3)AN(4)AC(12)
O(4)AN(5)AO(5)
O(5)AN(5)AC(14)
O(7)AN(6)AC(16)
N(3)AC(1)AN(1)
N(3)AH(3A)ꢀ ꢀ ꢀN(1)#2 0.86
N(2)AH(2)ꢀ ꢀ ꢀO(1)#3
0.86
0.82
3
O(3)AH(3)ꢀ ꢀ ꢀO(1)#3
1.65
1.97
2.03
2.14
2.01
2.07
2.15
2.447(3)
2.831(3)
2.887(4)
3.004(4)
2.841(4)
2.925(4)
3.004(4)
163.9
173.8
171.8
177.6
163.8
176.3
175.6
N(6)AH(6B)ꢀ ꢀ ꢀO(2)#3 0.86
N(6)AH(6A)ꢀ ꢀ ꢀN(2)#3 0.86
N(4)AH(4)ꢀ ꢀ ꢀN(1)#3
0.86
N(3)AH(3B)ꢀ ꢀ ꢀO(4)#4 0.86
N(3)AH(3A)ꢀ ꢀ ꢀN(5)#5 0.86
N(1)AH(1)ꢀ ꢀ ꢀN(4)#5
0.86
0.82
3
4
N(1)AC(9)
N(2)AC(14)
N(3)AC(9)
N(4)AC(23)
N(5)AC(23)
O(1)AC(1)
1.333(4)
1.334(4)
1.331(4)
1.377(4)
1.344(4)
1.278(3)
1.276(4)
119.5(3)
120.6(3)
124.9(3)
118.6(3)
114.9(3)
N(1)AC(13)
N(2)AC(13)
N(4)AC(19)
N(5)AC(24)
N(6)AC(19)
O(2)AC(1)
O(4)AC(5)
C(14)AN(2)AC(13)
C(24)AN(5)AC(23)
O(4)AC(5)AO(3)
N(2)AC(13)AN(1)
1.368(4)
1.351(4)
1.341(4)
1.339(4)
1.316(4)
1.229(3)
1.222(3)
118.0(3)
117.3(3)
124.6(3)
115.5(3)
O(3)AH(3)ꢀ ꢀ ꢀO(2)
1.57
2.04
1.99
2.13
2.13
2.07
2.383(4)
2.875(3)
2.848(3)
2.994(3)
2.925(3)
2.928(3)
167.7
163.2
175.1
178.0
153.1
176.8
N(6)AH(6B)ꢀ ꢀ ꢀO(4)#1 0.86
N(6)AH(6A)ꢀ ꢀ ꢀN(2)#2 0.86
N(4)AH(4)ꢀ ꢀ ꢀN(1)#2
0.86
N(3)AH(3B)ꢀ ꢀ ꢀO(3)#3 0.86
N(3)AH(3A)ꢀ ꢀ ꢀN(5)#4 0.86
O(3)AC(5)
C(9)AN(1)AC(13)
C(19)AN(4)AC(23)
O(2)AC(1)AO(1)
N(3)AC(9)AN(1)
N(5)AC(23)AN(4)
Symmetry transformations used to generate equivalent atoms for 1: #1 ꢁx + 1,
ꢁy + 2, ꢁz + 1; #2 x + 1/2, ꢁy + 3/2, zꢁ1/2. Symmetry transformations used to
generate equivalent atoms for 2: #1 ꢁx + 2, ꢁy + 1, ꢁz + 1; #2 ꢁx + 2, ꢁy, ꢁz + 1; #3
x, yꢁ1, z. Symmetry transformations used to generate equivalent atoms for 3: #3
x + 1, y, z; #4 xꢁ1, y + 1, z; #5 xꢁ1, y, z. Symmetry transformations used to generate
equivalent atoms for 4: #1 ꢁx, ꢁy, ꢁz + 1; #2 x, yꢁ1, z + 1; #3 x + 1, y + 1, z; #4 x,
y + 1, zꢁ1.
4
N(1)AC(9)
N(2)AC(14)
N(3)AC(9)
N(4)AC(23)
N(5)AC(24)
O(1)AC(1)
1.336(3)
1.332(3)
1.337(3)
1.381(3)
1.350(3)
1.213(4)
1.257(4)
117.7(2)
122.7(2)
121.8(4)
118.0(2)
120.3(2)
N(1)AC(13)
N(2)AC(13)
N(4)AC(19)
N(5)AC(23)
N(6)AC(19)
O(2)AC(1)
O(4)AC(2)
C(14)AN(2)AC(13)
C(23)AN(5)AC(24)
O(4)AC(2)AO(3)
N(2)AC(13)AN(1)
N(5)AC(23)AN(4)
1.375(3)
1.359(3)
1.345(3)
1.340(3)
1.313(3)
1.281(4)
1.224(4)
117.8(2)
116.4(2)
122.4(4)
114.5(2)
115.1(2)
3.2. Structural descriptions
O(3)AC(2)
3.2.1. X-ray structure of 5,7-dimethyl-1,8-naphthyridine-2-amine:
C(9)AN(1)AC(13)
C(19)AN(4)AC(23)
O(1)AC(1)AO(2)
N(1)AC(9)AN(3)
N(6)AC(19)AN(4)
(oxalic acid)0.5: H2Oꢀ[HLꢀ(ox2ꢁ
0.5ꢀH2O] (1)
)
The compound 1 of the composition [HLꢀ(ox2ꢁ
0.5ꢀH2O] was pre-
)
pared by reaction equal mol of 5,7-dimethyl-1,8-naphthyridine-2-
amine and oxalic acid, in which both protons of oxalic acid were
transferred to the N atoms on the naphthyridine ring. In the asym-
metric unit of 1 there existed one cation of 5,7-dimethyl-1,8-naph-
thyridinium-2-amine, half an anion of oxalate, and one water
molecule, as shown in Fig. 1. The CAO distances of the COOꢁ of
oxalate are ranging from 1.232(3) to 1.251(3) Å, which suggests
that the carboxylic group is deprotonated, and the N atom adjacent
to the amine group on the naphthyridine ring is protonated. The
significant difference in bond distances between O(1)AC(1)
(1.251(3) Å) and O(2)AC(1) (1.232(3) Å) in the carboxylate group
in compound 1 is caused by the fact that O(1) is involved in form-
ing more hydrogen bonds than that of O(2).
The very strong and broad features at approximately 3400–
3100 cmꢁ1 in the IR spectra of the four compounds arise from
OAH or NAH stretching frequencies. Aromatic and naphthyridic
ring stretching and bending are attributed to the medium intensity
bands in the regions of 1500–1630 cmꢁ1 and 600–750 cmꢁ1
,
respectively. Except for the above bands for 2, the bands at 1520
and 1320 cmꢁ1 were attributed to the
m
as(NO2) and s(NO2),
m
respectively [17]. The intense peak at ca. 1642 cmꢁ1 was derived
from the existence of the C@O stretches, and the band at ca.
1284 cmꢁ1 exhibited the presence of the CAO stretches of the car-
boxylate in compounds 1, 3, and 4 [18]. The absence of two broad
bands at ca. 2500 cmꢁ1 and 1900 cmꢁ1 which is characteristic of a
neutral OAHꢀ ꢀ ꢀN hydrogen-bond interaction was interpreted as a
lack of co-crystal formation in compounds 1, 2, 3, and 4 [19]. IR
spectroscopy has also proven to be useful for the recognition of
proton transfer compounds [20]. The most distinct feature in the
IR spectrum of proton transfer compounds are the presence of
strong asymmetrical and symmetrical carboxylate stretching fre-
quencies at 1550–1610 cmꢁ1 and 1300–1420 cmꢁ1 in compounds
1, 3, and 4, respectively [21].
The NAHꢀ ꢀ ꢀO hydrogen bond is formed between the naphthy-
ridinium cation and the oxygen atom of the oxalate, in which the
0
N(1)ꢀ ꢀ ꢀO(1) distance in these contacts is 2.656(3) ÅA, which is con-
siderably less than the sum of the van der Waals radii for N and
0
O (3.07 ÅA) [22]. Thus in the solid state, there is consistently ionic
hydrogen bonds formed between the naphthyridinium NH+ and
the oxalate ions, which is to be expected [23]. In compound 1,
there also exist strong electrostatic interactions between charged
cation units of NH+ and the di-anion oxalates.
In the framework of 1, there are 5,7-dimethyl-1,8-naphthyri-
dine-2-amine dimers formed through
adjacent parallel naphthyridine rings with the closest two rings
p–p interaction between

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S. Jin et al. / Journal of Molecular Structure 975 (2010) 128–136
Table 4
Character IR bands of chemical functionalities of the four organic salts 1–4.
Compound
m
(OH)
m
as(NH)
m
s(NH)
m
(C@O)
mas(COO)
m
s(COO)
m(CAO)
1
2
3
4
3360s (broad)
3306s
3440s
3340s
3440s (multiple)
3140s
3240s
3160s
3320s
1640s
1580s
1380s
1288s
3460m (broad)
3560s
1645s
1648s
1580s
1580s
1380s
1380s
1284m
1280s
Fig. 1. Molecular structure of 1 showing the atomic numbering scheme. Displacement ellipsoids are drawn at the 30% probability level.
distance of 3.386 Å. In addition there are also 1D chains formed by
5,7-dimethyl-1,8-naphthyridine-2-amine dimers through N+
AHꢀ ꢀ ꢀOꢁ hydrogen bonds between the N+AH of 5,7-dimethyl-1,8-
naphthyridinium-2-amine and the oxygen atom of the oxalate.
These chains extended along the directions that slipped from the
b and c axis direction with angles of about 67°, respectively. The
chains extending in the two crossed directions were connected
through the lattice water molecules to form 3D network structure,
which is shown in Fig. 2. The lattice water formed two hydrogen
bonds with the naphthyridine amine group and the ring N atom.
The lattice water also formed two H bonds with the oxalate in
which only one hydrogen atom of the water is involved in forming
two hydrogen bonds with the oxalate in bifurcate mode. In the
compound the oxalate formed eight hydrogen bonds, of which
Fig. 2. 3D network structure of 1 which is viewed along the c axis.

S. Jin et al. / Journal of Molecular Structure 975 (2010) 128–136
133
six hydrogen bonds connect with the naphthyridine moiety, and
two with the lattice water. Two oxygen atoms of the oxalate both
formed two hydrogen bonds with the naphthyridine in bifurcate
fashion. Each of the other two oxygen atoms formed one hydrogen
bond with the amine group of the naphthyridine, respectively.
The deprotonated carboxyl group acts as hydrogen bond accep-
tor for the formation of hydrogen bonds with protonated N atoms
on the naphthyridine rings, and the amine group also. The
hydrogen bond involves N(1) on naphthyridine rings and O(1) on
the COOꢁ carbonyl oxygen atom in which the naphthyridine-car-
boxylate moieties are almost coplanar (the torsion angle
O(1)AC(1)AO(2)AN(3) is 2.34 (2)°). In addition, there are also
CAHꢀ ꢀ ꢀO (CAO distance is 3.478 Å) hydrogen bonds between
crossed chains formed between 3ACH of naphthyridine and O
atom of oxalate.
ratio, which crystallizes as triclinic pale yellow crystals in the cen-
trosymmetric space group P-1. The structure of 2 with the atom-
numbering scheme is shown in Fig. 3. This is a salt where the OH
groups of 2,4,6-trinitrophenol are ionized by proton transfer to
one nitrogen atom (N(2)) of the naphthyridine moieties, which is
also confirmed by the bond distance of O(1)AC(11) (1.256(3) Å)
for phenolate (1.24 0.01 Å) [24]. In this case the less basic nitro-
gen atom close to the methyl group is protonated which is similar
to the published result [25], but it is different from the compound
1. In the compound, there is one ion pair without solvent mole-
cules, which is well agreement with the micro-analysis results.
Two naphthyridines formed dimers through intermolecular
hydrogen bonds of NAHꢀ ꢀ ꢀN. In the dimers two naphthyridinium
cations and two 2,4,6-trinitrophenolate anions self-assembled via
N+AHꢀ ꢀ ꢀOꢁ hydrogen bonds to form bis-2,4,6-trinitrophenolate
terminated moiety, in which the two terminated 2,4,6-trinitro-
phenolates exist in trans conformation. When viewed along the a
axis, the corresponding 2,4,6-trinitrophenolate ions in the adjacent
bis-2,4,6-trinitrophenolate terminated moieties are antiparallel.
The planes formed by the terminated 2,4,6-trinitrophenolates are
3.2.2. X-ray structure of 5,7-dimethyl-1,8-naphthyridine-2-amine:
(2,4,6-trinitrophenol) [HLꢀ(pic)] (2)
Salt 2 was prepared by reaction of a methanol solution of 2,4,6-
trinitrophenol and 5,7-dimethyl-1,8-naphthyridine-2-amine in 1:1
Fig. 3. The structure of 2, showing the atom-numbering scheme. Displacement ellipsoids were drawn at the 30% probability level.
Fig. 4. 2D grid structure of 2 viewed along the a axis.

134
S. Jin et al. / Journal of Molecular Structure 975 (2010) 128–136
almost perpendicular to the planes formed by the naphthyridine
dimers which are formed through hydrogen bonds.
viewed along the a axis. There are also CAHꢀ ꢀ ꢀO hydrogen bonds
between two picrates of which the O atom comes from NO2 of
one picrate of one closed loop, and the phenyl CAH comes from
picrate of another adjacent parallel closed loop. These 2D grids
were connected further by CAHꢀ ꢀ ꢀO, and OAO contacts to form
3D layer structure.
In the solid state, there are consistently hydrogen bonds formed
between the naphthyridinium NH+, and the 2,4,6-trinitrophenolate
ions. Each naphthyridinium NH+ forms one hydrogen bond with
the oxygen atom of the 2,4,6-trinitrophenolate ions. The
N+ AHꢀ ꢀ ꢀOꢁ bond angles and the Nꢀ ꢀ ꢀO and Hꢀ ꢀ ꢀO bond lengths
are consistent with the values reported from a statistical analysis
of NAHꢀ ꢀ ꢀO hydrogen bonds involving imidazolium residues [26],
and from a study of N+AHꢀ ꢀ ꢀOꢁ hydrogen bonding in salts of imid-
azole with monocarboxylic acids [27].
Two naphthyridine dimers and four picrates form closed loop
structure through CAHꢀ ꢀ ꢀO interaction. Adjacent closed loops con-
nect through OAO (NO2ANO2), and CAHꢀ ꢀ ꢀO (between 3ACH on
naphthyridine and NO2 of picrate with the CAO distance of
3.495 Å) interactions to form 2D grid structure (Fig. 4) when
3.2.3. X-ray structure of (5,7-dimethyl-1,8-naphthyridine-2-amine)2:
(terephthalic acid) [(HL)+ꢀLꢀ(tp)0.5ꢀ(H2tp)0.5] (3)
The asymmetric unit of 3 consists of one molecule of 5,7-di-
methyl-1,8-naphthyridine-2-amine, one cation of 5,7-dimethyl-
1,8-naphthyridin-1-ium-2-amine, half a molecule of terephthalic
acid, and half an anion of terephthalate, as shown in Fig. 5. The
CAO distances of COOꢁ of the terephthalate are ranging from
1.229(3) to 1.278(3) Å, which suggests that the carboxylate group
is partially deprotonated, while the N atom of 5,7-dimethyl-1,8-
Fig. 5. The structure of 3, showing the atom-numbering scheme. Displacement ellipsoids were drawn at the 30% probability level.
Fig. 6. 2D grid structure of 3 viewed from the c axis.

S. Jin et al. / Journal of Molecular Structure 975 (2010) 128–136
135
naphthyridinium ions suspended between the two chains through
NAHꢀ ꢀ ꢀO hydrogen bonds. The carboxylate O atom of one tere-
phthalate forms hydrogen bond with the carboxyl group of the
terephthalic acid in head to tail fashion. The OH group of the tere-
phthalic acid only acts as hydrogen bond donor for one O atom of
the carboxylate anion of the terephthalate, while another O atom
of the carboxylate functioned as hydrogen bond acceptor forms
one hydrogen bonds with the amine group of the naphthyridine.
The same as the carbonyl group in the terephthalic acid, the depro-
tonated carboxyl group in the terephthalate acts as hydrogen bond
acceptor for the amine group of the naphthyridine dimer also.
There are also CAHꢀ ꢀ ꢀO hydrogen bonds formed between the car-
bonyl group of the terephthalate and 3ACH of naphthyridine with
CAO distance of 3.557 Å. In compound 3, there do not exist strong
electrostatic interactions between charged cation units of NH+ and
the di-anion terephthalates. One-dimensional grid structure is
formed through naphthyridine dimers forming hydrogen bonds
to di-ion carboxylate groups of two parallel terephthalate chains
along the a axis, in which the naphthyridine dimers are almost per-
pendicular with the plane composing of two parallel terephthalate
chains (Fig. 6).
The terephthalate chains were also extended along the c axis
direction through the CAHꢀ ꢀ ꢀO (CAO distances are 3.549 and
3.409 Å respectively) interactions to form 2D grid structure. The
CAHꢀ ꢀ ꢀO interactions were formed between the phenyl CAH and
the oxygen atoms belonging to COOꢁ of terephthalate or OH group
of terephthalic acid, respectively. The naphthyridine dimers were
bound to the terephthalate grids through NAHꢀ ꢀ ꢀO and CAHꢀ ꢀ ꢀO
hydrogen bonds to form 3D ABAB layer structure when viewed
along the c axis direction.
Fig. 7. The structure of 4, showing the atom-numbering scheme. Displacement
ellipsoids were drawn at the 30% probability level.
naphthyridine-2-amine is partially protonated. The neutral
NAHꢀ ꢀ ꢀO hydrogen bond is formed between the oxygen atom of
the carboxylate and the amine group (N(6)ꢀ ꢀ ꢀO(2)#3, 2.831(3) Å),
there does not exist ionic N+AHꢀ ꢀ ꢀOꢁ hydrogen bonds. In the COOH
group, two CAO bond lengths are obviously different between
O(3)AC(5) (1.276(4) Å) and O(4)AC(5) (1.222(3) Å) which are also
confirming the reliability of adding H atoms experimentally by dif-
ferent electron density onto O atoms as mentioned above. But for
the COOꢁ group, two CAO bond lengths are basically not equal
with an average value of 1.2535 Å, which is shorter than that of
the single bond of O(3)AC(5) (1.276(4) Å) but longer than that of
the double bond of O(4)AC(5) (1.222(3) Å) in terephthalic acid.
This supports our assignment of the terephthalate anions. It is clear
that the difference in bond lengths of CAO within the carboxylic
acid group (0.054 Å) is almost the same with the one found in
the terephthalate anions (0.049 Å).
3.2.4. X-ray structure of (5,7-dimethyl-1,8-naphthyridine-2-amine)2:
(phthalic acid) [(HL)+ꢀLꢀ(Hopꢁ)] (4)
The crystal structure of 4 consists of one monoanion of
phthalic acid, one molecule of 5,7-dimethyl-1,8-naphthyridine-
2-amine, and one cation of 5,7-dimethyl-1,8-naphthyridin-1-
ium-2-amine in the asymmetric unit (Fig. 7). Only one proton
of the phthalic acid has transferred to the N atom of the naph-
thyridine ring. Of the two naphthyridines there is only one
naphthyridine that has been protonated, while the other naph-
thyridine remains neutral. The monoanion of phthalic acid has
In the structure, there are chains of terephthalate moieties
linked via COOꢁꢀ ꢀ ꢀCOOH hydrogen bonds along the a axis, with
Fig. 8. 3D network structure of 4 viewed along the c axis.

136
S. Jin et al. / Journal of Molecular Structure 975 (2010) 128–136
intramolecular hydrogen bonded structure through strong hydro-
gen bond S11(7) O(3)AH(3)ꢀ ꢀ ꢀO(2) interactions, which agrees well
with the reported results [28]. Two naphthyridines formed di-
mers through complementary hydrogen bonds of NAHꢀ ꢀ ꢀN. In
every dimer there is an anti-ion of hydrogen phthalate, which
forms two hydrogen bonds of Oꢀ ꢀ ꢀHAN with the amine group
of two different naphthyridine dimers. The two dimers further
Nos. 739,932 for 1, 746,307 for 2, 746,308 for 3, and 746,465 for
4. Copies of this information may be obtained free of charge from
the +44 1223 336 033 or email: deposit@ccdc.cam.ac.uk or
www: http://www.ccdc.cam.ac.uk.
Acknowledgment
formed donut structure through
closest approach of the ring centers between the naphthyridine
ring in adjacent parallel dimers is 2.994(3) Å. The adjacent do-
p–p interaction of which the
We are grateful for the financial support of the ZheJiang For-
estry University Science Foundation and the financial support from
MeiTe Industry CO., Limited, HangZhou, ZhengJiang and Rongkang
Industry CO., Limited, HangZhou.
nuts were further connected by 6-CHꢀ ꢀ ꢀ
p interaction (between
6ACH of naphthyridine and the phthalate ring) along the b axis
to form a 1D chain structure, in addition the donuts along the a
axis were connected through CAHꢀ ꢀ ꢀO hydrogen bonds between
the carbonyl group of phthalate and the 4ACH of the naphthyri-
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4. Conclusions
A series of organic salts with different topologies have been syn-
thesized and structurally characterized. The different hydrogen
bond interaction modes of the carboxylate anions and 5,7-di-
methyl-1,8-naphthyridinium-2-amine cation lead to a wide range
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5. Supporting information available
Crystallographic data for the structural analysis have been
deposited with the Cambridge Crystallographic data center, CCDC
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